The invention relates generally to centrifugal pellet dryers utilized to dry plastic pellets which have been cut from strands of plastic by a pelletizer, and more particularly, to a centrifugal pellet dryer apparatus including improved rotor and lifters which can provide enhanced dewatering capabilities. The invention also relates to additional improvements in the centrifugal pellet dryer apparatus which can further enhance dewatering capabilities.
Generally, a water slurry of plastic pellets is introduced into a pellet dryer for separation of the pellets from the water. The dry pellets can then be conveyed to a shipping container or to a location for further processing. Centrifugal pellet dryers are well known in the art. In particular, U.S. Pat. No. 5,611,150, to Yore, Jr., issued Mar. 18, 1997, which is hereby incorporated herein by reference, discloses a xe2x80x9cCentrifugal Pellet Dryerxe2x80x9d wherein a slurry of pellets and water is introduced upwardly through the bottom of the dryer, into a hollow region in the rotor and out through ports in the rotor. In this manner, the slurry is directed radially outwards from generally the center of the rotor. The rotor has lifters, i.e. blades, which carry the pellets and water upwardly through the dryer as the water is forced outwardly by centrifugal force through a cylindrical screen that surrounds the rotor. The dewatered pellets are discharged through an exit port near the top of the cylindrical screen while the separated water is drained through the bottom of the dryer housing. Prior to the centrifugal pellet dryer described in the ""150 patent, centrifugal pellet dryers introduced the slurry of pellets and water through an entry port in the side of the pellet dryer. The slurry was injected inwardly towards the rotor and the lifter affixed to the outside of the rotor drove the slurry outwards against the screen as the rotor was rotated. In that configuration, the rotor breaks the stream of the slurry where it is introduced through the side of the pellet dryer, causing a large portion of the pellets to be deposited on the portion of the screen nearest the slurry entry port. In contrast, the pellet dryer disclosed in the ""150 patent describes introducing the slurry of pellets and water through the center of the rotor and radially outward into the space between the rotor and the mesh member. By introducing the pellets and water through the center of the rotor, the slurry of pellets and water is radially, and more evenly, distributed by the rotor lifters about the interior of the pellet dryer. Thus, pellet impacts are not concentrated at any single area of the screen, in contrast to the conventional pellet dryers which injected slurry through a side port where most of the initial dewatering is done in the first quadrant adjacent to the slurry inlet, which loads up the screen and limits its effectiveness in the initial dewatering stage.
Introducing the slurry through the rotor also minimizes radial bearing loads that are present in the prior-pellet dryers since the lifter blades in those pellet dryers must break the stream of water and pellets in a single location which is off-center from the axis of the rotor. The feed of the slurry through center of the rotor also enhances the centrifugal force of the rotor to throw the water radially through the screen to thereby enhance the initial dewatering stage of the dryer.
Recently issued U.S. Pat. No. 5,987,769, to Ackerman, et al., issued Nov. 23, 1999, discloses an alternative manner of introducing a slurry of pellets and water generally into the center of the pellet dryer. However, the slurry is not injected radially outwards through the rotor as in the ""150 patent. Rather, the slurry is injected axially upwards onto a lower face of the rotor. Lifters, or blades, are provided on a lower face of the rotor which throw the slurry radially outwards against the cylindrical screen. A disadvantage of this pellet dryer can be that the lower end of the rotor is not supported. This is because the slurry is directed upwards against the lower face of the rotor. Thus, the rotor is supported only at the top of the dryer, which can be disadvantageous since it is desirable to support a rotating member like the rotor at both ends to maintain axial alignment. Supporting the rotor at only one end can create problems with balance and vibration when the rotor is rotated, especially when rotated under a load as when slurry is being pumped by the rotor through the pellet dryer. Accordingly, it can generally be desirable to support the rotor at both ends to improve efficiency and longevity of the dryer.
Conventionally, it was generally believed that dewatering was best accomplished by impacting the pellets against the dewatering screen to remove the water. However, it has been discovered that dewatering can best be achieved by instead limiting the number of impacts between the pellets and the dewatering screen and increasing the number of impacts between the pellets and the lifters. Residual water on the screen can actually be reacquired by pellets through repeated impacts with the screen. The slurry of pellets and water can typically be introduced into the pellet dryer while the pellets are still hot. The internal heat of the pellets actually assists in the drying process. An additional reason why pellet impacts against the dewatering screen can be disadvantageous is that the screen acts like a xe2x80x9cgrater,xe2x80x9d shaving or breaking off pieces of the pellets during impacts. These pieces of pellets, commonly called xe2x80x9cfines,xe2x80x9d can cause other problems with the operation of the pellet dryer and with disposal of the water removed from the slurry. Consequently, lifters which reduce the number of impacts with the dewatering screens can improve the drying efficiency and reduce fines. Lifters can be designed to reduce the number of impacts with the dewatering screens by creating the lifters with an inwardly curved surface which tends to direct the pellets away from the dewatering screen and back in towards the rotor surface and other lifters. This additionally results in increased impacts between the lifters and the pellets.
In prior pellet dryers, generally flat lifter blades are attached to the surface of the rotor in a helix configuration at a 45 degree angle. The 45 degree angle is what xe2x80x9cliftsxe2x80x9d the pellets upwards through the pellet dryer. The lifters are generally flat in that there is no curvature apart from the helix curvature imparted as a result of attachment to the cylindrical rotor. The flat lifters direct the pellets out into the dewatering screen. The lifters are also conventionally attached to the surface of the rotor in an evenly distributed manner, in that there are the same amount of lifters in each row along the entire length of the rotor. Prior art pellet dryers typically use 5-6 lifters evenly spaced radially around the circumference of the rotor. The lifters are also conventionally aligned horizontally in rows and vertically in columns.
Accordingly, there is a need for a lifter, rotor and pellet dryer apparatus which provides improved dewatering capabilities through increased pellet impacts with the lifters and reduced pellet impacts with the dewatering screen.
According to the invention, a centrifugal pellet dryer apparatus, rotor and lifter is provided wherein the lifters can have a front surface configured to deflect pellets inwardly toward the rotor surface and other lifters, and the rotor can have lifters attached in a configuration designed to increase pellet impacts with lifter blades as well as providing different regions of lifters wherein different numbers of lifters can be provided in the different regions of lifters along the length of the rotor. The specially configured surface of the lifters tend to control the pellet path, keeping the pellets in the lifter envelope and away from the dewatering screens. The configuration of the lifters on the rotor, generally a higher concentration of lifters on the lower region of the rotor can further create increased pellet impacts with the lifters.
The centrifugal pellet dryer can have an outer housing with a water removal port formed in the outer housing and one or more mesh members disposed vertically within the outer housing. The mesh member can be formed of material that permits passage of water while blocking the passage of pellets therethrough. A vertically disposed rotor with lifters affixed thereto is journaled for rotation coaxially within the dewatering screen to direct pellets upwardly to a pellet discharge port in an upper portion of the dryer. The rotor can have a hollow interior portion coaxial with the rotor and a plurality of radial passages extending between the hollow interior portion and the outer surface of the rotor. A slurry inlet is provided adjacent the bottom of the rotor in communication with the hollow interior portion of the rotor. Slurry introduced through the slurry inlet is conducted through the hollow interior portion of the rotor and through the radial passages which direct the slurry into a space between the outer surface of the rotor and the dewatering screen.
The lifters, also called blades, are provided on the outer surface of the rotor in a helix configuration and are angled approximately 45 degrees which xe2x80x9cliftsxe2x80x9d the pellets upwardly toward the pellet discharge port. To reduce pellet impacts with the screen, and to increase pellet impacts with the lifters, the surface of the lifters can be configured to direct pellets inwardly towards the rotor and other lifters, and away from the dewatering screen. The number or rows, the number of lifters in each row provided in different regions along the length of the rotor, and spacing/alignment of the lifters affixed to the outer surface of the rotor can be designed to further increase the number of impacts between the lifters and the pellets.
The centrifugal pellet dryer can also be internally chamberized into multiple sections for example: a slurry inlet and initial dewatering section; a secondary dewatering and drying section; and a pellet discharge section. In this three section embodiment, each section can employ a dewatering screen, which may extend for all or only a part of each section.
An exhaust port through the top of the housing can extend into the upper and middle sections for creating a negative pressure, i.e., a vacuum, in those sections to enhance a vertical counter flow of air through the upper and middle sections to pull moisture down from the discharge section. A separator plate can be provided between each section to somewhat seal each section to enhance the effect of the negative pressure to inhibit the upward movement of moisture into the upper drying and discharge sections. A hole can be provided through the separator plates in the upper and middle sections for creating a negative pressure in those sections via the exhaust port. A water seal can be created at the water outlet ports to create an air tight water seal, and the slurry inlet tube can be elbow-shaped to provide a water trap preventing air from entering through the slurry inlet. The water seal and water trap can inhibit air from entering the lower section and moisture laden air from rising up through the dryer.
The centrifugal pellet dryer can also be designed so that the different sections are modular and can thus be interchangeable, as well as for convenience of assembly and disassembly of the dryer. The modular sections can also be designed such that the size of the pellet dryer, with respect to the length, can be varied for a given diameter pellet dryer simply by varying the number of secondary dewatering sections provided between the slurry inlet and pellet discharge sections. The first and last sections would remain the same for a given diameter pellet dryer. However, the length of the rotor would have to be different to accommodate the changing length resulting from the addition or subtraction of secondary dewatering sections.
Portions of the rotor can be provided with different lifter configurations associated with the different sections of the dryer. For example, the lower portion of the rotor which is disposed within the slurry entry and initial dewatering section can be provided with a greater number, and different configuration, of lifters designed to initially separate most of the water out of the slurry. The middle portion of the rotor associated with the secondary dewatering and drying sections can have fewer lifters and can be differently configured. Since most of the water has already been removed in the first section, only the minimum number blades required to maintain an upward travel of the pellets toward the discharge portion are needed in the middle sections.
An air inlet, preferably tangential, communicating the atmosphere with the pellet discharge port can be provided to permit air to be drawn from the atmosphere as the pellets are discharged. The counter-flow air thus created can eliminate the creation of a disadvantageous flow of air drawn through the pellet discharge port which could otherwise occur.
Other features can include a lower bearing for the rotor which runs in water. Failure of the lower rotor bearing due to contamination by water from the slurry is a known problem with conventional centrifugal pellet dryers. Thus, a water-cooled bearing, typically made from a type of plastic, can eliminate such bearing failure problems. For ease of cleaning and maintenance, the outer housing and the screen can be segmented such that they can be along a side and removed from around the rotor. Additionally, the motor can be located directly. above the rotor to drive the rotor directly, thus eliminating belts and sheaves that produce radial loads on the rotor bearings. This can result in longer lasting bearings as well as eliminating belts and sheaves which wear out and must be regularly replaced.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings figures of certain embodiments thereof.